A Russian fighter flew dangerously close to a US warplane over eastern Syria, US defense officials said Friday, highlighting the risks of a serious mishap in the increasingly crowded airspace.

The near miss occurred late on October 17, when a Russian jet that was escorting a larger spy plane maneuvered in the vicinity of an American warplane, Air Force Lieutenant General Jeff Harrigan said.

The Russian jet came to 'inside of half a mile', he added.

A Russian fighter flew dangerously close to a US warplane over eastern Syria on October 17. Here Russian Sukhoi Su-25 ground attack aircraft take off from the Hmeimim military base in Latakia province, Syria on March 16

The near-miss took place in Eastern Syria. Russia often transits through the area as part of its constant air patrols over Aleppo

Another US military official, speaking on condition of anonymity , said the American pilot could feel the turbulence produced by the Russian jet's engines.

'It was close enough you could feel the jet wash of the plane passing by,' the official said.

It appeared the Russian pilot had simply not seen the US jet, as it was dark and the planes were flying without lights.

The US-led coalition has set up a hotline with Russian counterparts so the different militaries can discuss the approximate locations and missions of planes, and avoid operating in the same space at the same time.

In this case, the American pilot tried unsuccessfully to reach the Russian jet via an emergency radio channel.

The next day, US officers used the hotline to ask Russia what had happened and they said 'the pilot didn't see' the American plane, the official said.

Harrigan said there had been an increase in close calls over the past six weeks, with intentional near misses -- when a Russian jet deliberately follows a coalition plane too closely -- 'happening one every 10 days-ish'.

The rubble of destroyed buildings is seen in Aleppo. Russian planes fly over the city as part of air patrols, and the military has asked to be allowed to resume airstrikes (file image)

Russia is flying constant air patrols over Syria, the vast majority of them over the devastated city of Aleppo, and routinely transits parts of the country the US-coalition operates in, officials said.

Russia says it has not bombed Aleppo since October 18.

The Pentagon periodically chides Russia for 'unsafe and unprofessional behavior' in air operations.

This incident was deemed unsafe, but not necessarily unprofessional, officials said.